Field of the Invention
The present invention relates to vertical axis lift-driven wind turbines, and more particularly, to vertical axis wind turbines with blade configurations that ameliorate gravitational and centrifugal forces.
Description of Related Art
Wind energy is an appealing source of renewable energy, and horizontal axis wind turbines (HAWT) have emerged as the predominant wind turbine configuration over the past 30 years and even up to the present, primarily due to advantages in rotor costs for turbines with power generating capacities less than 5 MW. However, HAWTs have shortcomings as well. They require a control mechanism to keep them pointed into the prevailing wind. They are subject to fatigue because the rotor blades are cantilevered from a horizontal generator axis, which subjects them to regular, periodic stresses due to gravity as they rotate. Fatigue issues are aggravated as the blades get longer and heavier for turbines with higher power ratings. Longer rotor blades are also more susceptible to aeroelastic effects, which further contribute to fatigue and can reduce blade life even more. To counter the effects of fatigue, the blades, and the HAWT tower carrying them, must be made more robust, which uses more material than would otherwise be necessary and thus increases cost.
Another disadvantage of HAWTs is that the blade roots have to be mounted at the top of a tower, sometimes hundreds of feet tall, where they are connected to a generator. This increases the cost of maintenance and repairs to the generator and its associated machinery. The elevated location of the generator also complicates HAWT installation offshore because it presents problems in anchoring the base of the tower, even one of modest height. Moreover, anchoring a top heavy HAWT tower to the seabed in relatively shallow water still requires expensive infrastructure that is typically not needed for land-based installations, and is even more challenging in deep water because the tower must be supported by a free-floating platform. The problems encountered with offshore installations, particularly in deep water locations, are obviously exacerbated as the HAWT is made larger and taller, and thus heavier, to increase power generating capacity.
A vertical axis wind turbine (VAWT) can avoid many of these problems. There are myriad VAWT configurations, some of which are discussed in Whitehouse, Glen R., et al., “Variable Geometry Wind Turbine for Performance Enhancement, Improved Stability and Reduced Cost of Energy,” Wind Energy, John Wiley & Sons, Ltd. published online at http://onlinelibrary.wiley.com/doi/10.1002/we.1764/full (May 15, 2014). One type of prior art VAWT that can be adapted for application of the present invention is depicted in FIG. 1. This is a simplified representation of what is sometimes referred to as an “H-rotor” design, in which each of a pair of blades 1 is mounted to the outer end of a respective strut 2 to form the characteristic “H” shape of this design. The inner ends of the struts are attached at the top of a rotating vertical shaft 3 inside a tower 4, which is supported by a frame 5. The shaft 3 is attached to a generator 6 at the base of the tower for generating electricity as the shaft rotates. There are numerous variations on this concept, with various blade mounting arrangements and adjustable blade orientations. See, for example, U.S. Pat. No. 1,835,018, No. 4,105,363, No. 4,204,805, No. 4,293,279, No. 4,325,674 (FIG. 7), No. 6,784,566, No. 6,974,309, No. 7,677,862, and No. 8,322,989, and Korean Pub. No. 10-2009-0112469.
FIGS. 2 and 3 illustrate the manner by which a lift-driven VAWT rotates a shaft in the presence of a prevailing wind UW. (Other types of VAWTs can be classified as “drag-driven,” which work on the same principle as the familiar cup-shaped anemometer impeller.) FIG. 2 illustrates that a rotor blade 1 rotates about an axis A of the shaft 3 in the direction of the arrow drawn around the axis. FIG. 3 illustrates notionally how lift generated by the blades in the presence of a prevailing wind creates a rotational force on the shaft 3. For the sake of illustration, the blades in FIG. 3 are depicted at a point in their travel about the axis A where the prevailing wind vector is perpendicular to the blade chord c with a velocity of UW. Due to the rotation of the blade, there is also an airflow vector parallel to the blade path that corresponds to the tangential velocity VT of the blade. If the blade has no twist and wake-induced effects are discounted, the resulting airflow velocity VR approaches the blade at an angle of attack α relative to the chord c. (A further simplification assumes that there is no structural deformation of the blade.) According to known principles of aerodynamics, the blade 1 generates lift primarily as a function of α and VR. As seen in FIG. 3, the resulting notional lift vector will have a component in a direction tangential to the path of the blade 1 that causes the shaft 3 to rotate about the axis A.
The construction of a VAWT of this general configuration has a number of advantages over an HAWT, a principal one being that it is independent of the direction of the prevailing wind. The turbine blades are also comparatively straightforward to design and manufacture because of their relatively simple geometry as compared to HAWT blade airfoils that twist and change chord along their span. VAWT rotor blades can be “furled,” by folding the struts and blades inwardly toward the axis of rotation, to minimize the possibility of damage caused by excessive wind velocities during storms. They are not subject to periodic bending stresses due to gravity as they rotate, which theoretically permits VAWTs to be scaled up to very large sizes. VAWT power generating machinery is located at its base, which together with a potentially larger power generating capacity, makes VAWTs good candidates for mounting on floating platforms in deep water because they can be made large enough to generate significant amounts of electricity and be sited far enough offshore so they cannot be seen from coastal areas in spite of their size. These and other advantages of VAWTs for offshore installation are discussed in more detail in Paquette, Joshua, et al., “Innovative Offshore Vertical-Axis Wind Turbine Rotor Project.” Proc. of European Wind Energy Assoc., Copenhagen, Denmark, Apr. 16-19, 2012.
However, lift-driven VAWTs with this type of H-rotor configuration (cantilevered, generally vertical blades rotating about a central axis) present their own design challenges. For one thing, the rotating blades are subject to the centrifugal forces CFP shown in FIG. 2, which increase as the blades are made larger and thus heavier. Moreover, the horizontally directed lift generated by the blades varies as they revolve about the axis and present periodically varying angles of attack to the prevailing wind, thus subjecting them to periodic lift forces that can cause the blades to fatigue. Making the blades stronger to resist fatigue failure means also making them heavier, so that the struts supporting the blades against the force of gravity must also be made stronger and thus heavier and more costly. Additional struts can be used, but that also adds to weight and cost, and complicates the design, particularly if it is desired to incorporate mechanisms to furl the rotor blades.
Large (utility-scale) VAWT technology is not as mature as that for HAWTs, with no systems being offered or produced by existing utility-scale turbine manufacturers. VAWTs produced in the 1980s that were considered utility-scale at the time are too small to be considered as such (by a factor often or more) by current standards. As a result, VAWTs have not found widespread acceptance for utility scale power generation facilities because a practicable VAWT must be made large enough to approach or exceed the power generating capacity per unit cost (including design, construction, and installation) of a comparable HAWT with the same capacity.
Traditional approaches to engineering wind turbines have treated aerodynamic and structural design independently, by settling first on an aerodynamic design that maximizes power generating efficiency, and then designing the structure necessary to take the resulting loads. This often results in large, costly structures, which may not even be feasible with currently known materials, manufacturing technologies, and construction techniques when applied to the next generation of offshore designs. However, there are some examples of departures from the traditional design approach. U.S. Pat. No. 4,293,279 describes a modified H-rotor-type VAWT system with an oval ring-like blade in which the ring shape is designed to cancel the bending moment on the blade itself due to centrifugal force, but it does not address the effects of centrifugal forces on the rest of the structure, nor does it address any design issues relating to weight. U.S. Pat. No. 4,561,826 describes a number of VAWT configurations with counter balanced cantilevered blades that pivot and gimbal on the top of a tower. In this design, the resulting blade angle of inclination for a given wind speed is established when equilibrium is reached among aerodynamic, gravitational, and centrifugal forces on the blade. However, this only addresses operational features and does not solve problems, including those discussed above, that have thus far presented a barrier to scaling up VAWTs to sizes which will be more feasible for a wide variety of commercially applications. Finally, U.S. Pat. No. 8,083,383 discloses a VAWT with blades attached to inclined struts mounted to a base at an angle that form a V-shaped rotor. The blades are mounted at angles designed to reduce the overturning/tipping moment on the structure and thus improve stability, but this patent does not address structural design problems encountered when attempting to scale up VAWTs to the large sizes required to compete effectively with the presently well entrenched, widespread use of HAWTs.